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890GX/HD4290 dual display question

I'm looking at mainboards with this chipset. Virtually all of them have 3 display connectors on the back -- DSUB, DVI, and HDMI.

Now here's the problem... all the manuals for all of them say that you can't use the DVI and the HDMI at the same time. Which is REALLY STUPID.

So the question is this:
Is it a HARDWARE limitation? A DRIVER limitation? Or is it a COMPLETE FABRICATION? I.e., the LATTER TWO both wouldn't apply to the open source drivers.

It seems very odd to me that there WOULD BE both a DVI and an HDMI plug on it when they are actually quite the same thing -- it would make more sense to give it JUST a DVI plug, since HDMI maps directly to DVI using a trivial adapter.

So a little clarification is needed here.... Anyone help?
No GUESSES please, and no copy/paste from the manual, I know what it says.

It's a hardware limitation. Most motherboards use the same encoder for both DVI and HDMI, so you can use one or the other. The oems put both connectors on there since it gives the end user the most options as far as what kind of connector they want to use.

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remember that any pre-evergreen card cannot drive more than two displays at a time, anyway. To drive two external displays, you'll need to turn off the internal panel - apparently a rare use-case most vendors don't care about.

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RS780/RS880 cards have two digital transmitters: LVTMA and PCIEPHY. PCIEPHY uses pcie lanes so a lot of oems don't use it on motherboards since it means giving up a pcie x16 slot or having to add a bios option to toggle the pcie x16 slot or the secondary digital port. PCIEPHY is more commonly used on laptops for HDMI ports since you don't need the pcie x16 slot. There are a few vendors that use PCIEPHY on desktop motherboards but the much more common case is DVI and HDMI on LVTMA.

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remember that any pre-evergreen card cannot drive more than two displays at a time, anyway. To drive two external displays, you'll need to turn off the internal panel - apparently a rare use-case most vendors don't care about.

The problem here is that there's no sensible way to drive two displays, and analog with the wavy flickery lines is no option.

@agd5f: thanks, guess an add-on card it is then. Which doesn't solve anything since it means eating up PCIe lanes anyway. That bios toggle option seems like a real good idea. It is unbelievable that nobody seems interested in implementing this option since most people don't use ANY add-on PCIe *except* possibly for a display card. I guess they're too interested in crippling their mainboards to boost add-on card sales.

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If you are a gamer or power user, you'll probably add in a x16 card anyway. If you are using the onboard chip for htpc or desktop use, you probably aren't using two monitors much less two digital monitors. Most users still only use one monitor. The current situation makes the most sense to oems since it gives you the option of adding in an x16 card without having to mess with a bios option and it avoids the expense of wiring in the second digital port to share the pcie lanes that will probably not get used in most cases anyway. That said, there are a few boards that offer dual digital on the integrated card, but most do not. Some examples:
ASUS M3A78-EM
ASUS M2A-VM HDMI
ASRock A780FullDisplayPort

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If you are a gamer or power user, you'll probably add in a x16 card anyway. If you are using the onboard chip for htpc or desktop use, you probably aren't using two monitors much less two digital monitors. Most users still only use one monitor. The current situation makes the most sense to oems since it gives you the option of adding in an x16 card without having to mess with a bios option and it avoids the expense of wiring in the second digital port to share the pcie lanes that will probably not get used in most cases anyway. That said, there are a few boards that offer dual digital on the integrated card, but most do not. Some examples:
ASUS M3A78-EM
ASUS M2A-VM HDMI
ASRock A780FullDisplayPort

As a power user I have ZERO interest in an add-on card.
And for a gamer, IGP is pretty much pointless, so they certainly wouldn't be interested in having IGP at all.

And FYI: The M3A78-EM also doesn't do dual digital, except possibly for display port... which is incompatible with everything. That and the lack of DDR3 support drops that board off the list.

Anyway, the point is basically that your idea of having a bios setting (or even a jumper on the board) to reassign the PCIe lanes to PCIEPHY makes the most sense. If somebody DOES want dual digital display right off the IGP, it is an option. If they want an add-in card, the PCIe lanes aren't wasted.

As its going to end up happening though, I'm forced to drop an extra $40 on 4350, and you can bet that it won't be from anyone who also manufactures mainboards.

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My admin system has DP and VGA output from the IGP; the display has DVI, HDMI and VGA input, and the display res is too high for VGA to deliver useable quality.... so I'm running with a passive DP to DVI dongle and it seems to work fine.